Near-field evolution and mixing of a negatively buoyant jet consisting of brine from a desalination plant

Abstract: The aim of this study was to investigate the behavior of a dense jet discharged into lighter ambient water. This situation can be found for brine water discharged from desalination plants. Desalination plants discharge heavier brine water, generated in the process of manufacturing fresh water, to the receiving water, usually the sea. Future extended use of desalination technologies implies larger quantities of saline water discharge to the environment. For that reason, it is of importance to study the mixing processes to avoid large concentration of salt, which may produce environmental problems. The most efficient method to increase the dilution rate of the discharged water into the sea is by using a negatively buoyant jet. The design of this type of jet involves a wide range of variables. In this study a mathematical model was developed to simulate the jet behavior in order to determine the optimum discharge conditions for different scenarios. The governing equations for a buoyant jet were employed in the model, including mass conservation for water and salt, and two momentum equations. In addition, several assumptions were introduced to simplify the model, for example, self similarity for the velocity and concentration profiles. The mathematical model was compared with data from previous experimental studies as well as from a new experiment performed within the present study. In the model simulations performed in this study to reproduce the experimental runs it was difficult to observe a close relationship between the model and the experimental data for all experimental runs. For certain parameter ranges (i.e., salinity, nozzle angle, nozzle diameter), model predictions were satisfactory. However, looking at different ranges in the densimetric Froude number there were always some runs that displayed larger discrepancies between the model simulations and the data obtained in the experiment. A main conclusion of this study is that one or several of the assumptions used to derive the mathematical model is not satisfied in the experiment and some modifications of the equations derived are needed. Most likely a modified description of the entrainment coefficient is required that introduces a dependence on salinity and other parameters.